Camouflage material which is presently being produced for military use, particularly in the United States, has been developed to the stage at which it is capable of defeating most detection devices. However, radar devices capable of transmitting over different wavelengths are being developed. Coatings have been developed which are capable of presenting a visual appearance closely resembling any of a number of possible environments in which the camouflage is to be used, e.g., woodland regions, snowy regions, desert areas, and the like. In any or all of these environments, it is frequently desirable to also use the camouflage to provide suitable shielding against electromagnetic radiation, such as for example that used by radar or infrared (I.R.) devices, or the combination of the two. The term, "radar device", is used to include an apparatus capable of transmitting and receiving electromagnetic energy in any one of a number of wavelengths or bands including visual and radio broadcast waves.
If the material to be developed into a camouflage structure is to be radar defeating, a common practice is to provide a substrate with a plurality of electrically conductive fibers or fibrils, these being typically either metal, such as stainless steel, or elemental carbon in the form of graphite fibers. A common material now being used includes a spun-bonded, non-woven fabric wherein one surface has adhered a plurality of randomly oriented metal fibrils. The non-woven fabric with the fibrils attached is then coated or laminated, usually on both sides, with a film or layer of a polymeric material, commonly polyvinyl chloride. The polyvinyl chloride (PVC) itself may be impregnated or filled with pigment, as disclosed in U.S. Pat. Nos. 4,034,375 and 4,435,465, to impart to the resulting material the desired responses in the visible or near visible electromagnetic radiation spectra, particularly visible, infrared and ultraviolet regions. Alternatively, the polyvinyl chloride can be further coated with a pigment-containing coating or paint to achieve a desired optical response characteristic.
While the resulting product is generally suitable as a camouflage structure, certain disadvantages have appeared. One of these is that the radar reflectance characteristics initially built into the camouflage structure by virtue of the random disposition of metal fibers has a tendency to change when the finished camouflage structure is handled, crinkled, folded, or otherwise flexed in normal usage. The reason for this change in radar characteristics is not fully understood, but it has been established that different characteristics appear in the vicinity of the folds where broken particles accumulate and that, as a result, the camouflage can be distinguished by suitable radar analysis from the surrounding environment. Also, breakage of the semiconductors or other conductive materials creates a change in continuity and contact resistance so that there are absorption changes that may result in detection.
U.S. Pat. No. 4,495,239, discloses a camouflage structure, effective in the spectral range from visible light to radar waves, which comprises a base layer coated with a homogeneous metal layer reflective in the range of terrestrial thermal radiation as well as in the radar region of the spectrum (3 megahertz to 3,000 megahertz). The structure also has a surface resistivity of not more than 0.5 to 10 ohms per square cm and a subsequent coating of a camouflage paint containing pigments having reflective properties in the visible and near IR spectral regions that are similar to the natural background. A binder having high transparency in the 3-5 microns (.mu.m) and 8-14 .mu.m atmospheric windows of the far infrared, region of the spectrum is provided. The paint is applied so that its emissivity in those regions will vary over the surface of the material.
British Patent No. 1,605,131, discloses thermally structured camouflage materials having a surface which is highly reflective in the far infrared region of 3-5 .mu.m and 8-14 .mu.m. The structure has a coating of a camouflage paint containing a pigment having camouflage properties in the visible and near IR range. The structure further contains a binding agent having an emissivity less than 90% in the 3-5 .mu.m and 8-14 .mu.m range. The emissive power is structured in various ways so that it varies over the surface of the structure.
U.S. Pat. No. 3,733,606 addresses the problem of detection by radar by using a camouflage structure consisting of a multi-layered material for absorbing and reflecting signals for defeating radar waves transmitted over different frequencies. At least one layer is a thin, non-homogeneous electrically conducting film and the other layers assist in providing both a two dimensional and three dimensional effect.
U.S. Pat. No. 4,479,994 discloses a flexible, quilt-like multispectral camouflage blanket which functions as a radar absorber and also suppresses thermal and acoustical energy. The fabric material utilized is KEVLAR, polyvinyl chloride or SCRIM textiles.
U.S. Pat. No. 4,837,076 to Mc Cullough et al, which is herein incorporated by reference, discloses a class of carbonaceous fibers having the different degrees of electrical conductivity which may be used in the present invention.
U.S. Pat. No. 4,857,394, to Mc Cullough et al, which is herein incorporated by reference, discloses a class of fluorinated carbonaceous fibers which may be utilized in the camouflage structures of the invention.
It is understood that the term "web" as used herein is intended to define a textile fabric which for simplicity is intended to include knitted, woven and non-woven textile materials mats, battings, laminates, and the like.
The term "structure" as herein utilized is intended to mean a construction or arrangement of one or more physical or textile elements or materials into a complex entity, for example camouflage blankets, tents, webbing, nets and the like.
The term "carbonaceous materials" include carbonaceous fibers, foams, sheets, films or the like having a carbon content of greater than 65%.
The term "graphitic" as used herein relates to those carbonaceous materials having an elemental carbon content of at least about 92%, preferably, about 98%, and as further defined in U.S. Pat. No. 4,005,183 to Singer, which is herewith incorporated by reference.
It is to be understood that the percentage stated relate to percent by weight of the total composition unless stated otherwise.